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A stretchable conductor typically consists of two components: an [[elastomeric]] insulator and an [[electrical conductor]]. There are several approaches to producing stretchable and electrical conducting materials that fall into two categories: [[structural design]] and material innovation. [[File:Stretchable Microelectrode Array (sMEA) before and during stretch.png|thumb|left| sMEA before and during stretch]]
===Material
* '''Electronic Fillers''': This is the oldest approach to making an [[elastomeric]] material elastically stretchable. In principle, rigid and [[electrically conductive]] materials and mixed with an elastomeric [[polymer]] before curing to create stretchable composites. If the concentration of the electrically conductive filler is high enough they form a mesh-like [[percolation]] network that facilitates the free movement of charge carriers (ions, electrons) through contact junctions. The minimum concentration of the electronic filler material that is required to create conductive pathways for [[charge carrier]] transport through the elastomer<ref>{{cite journal |last1=Kyrylyuk |first1=Andriy V. |last2=van der Schoot |first2=Paul |title=Continuum percolation of carbon nanotubes in polymeric and colloidal media |journal=Proceedings of the National Academy of Sciences |date=17 June 2008 |volume=105 |issue=24 |pages=8221–8226 |doi=10.1073/pnas.0711449105 |doi-access=free |pmid=18550818 |pmc=2448818 }}</ref> is called the [[percolation threshold]].<ref>{{cite book |doi=10.1016/B978-1-895198-95-9.50011-X |chapter=Structure and Distribution of Non-Migrating Antistatics |title=Handbook of Antistatics |date=2016 |pages=117–127 |isbn=978-1-895198-95-9 |editor1-first=Jürgen |editor1-last=Pionteck |editor2-first=George |editor2-last=Wypych }}</ref> The [[percolation threshold]] is usually indicated as weight percentage (wt%) or volume percentage (vol%) of the filler material, and ranges from less than 1wt% for high aspect ration carbon [[nanotubes]] to over 15wt%. The type of filler materials ranges from metals in powder or [[nanowire]] form, [[carbon]] as [[graphite]] or [[nanotubes]], to electrically conducting polymers.
* '''‘Wavy’ [[Nanowires]] and Nanoribbons''': The spontaneous formation of wavy patterns of aligned [[buckles]] that is caused by the deposition of a thin gold film on the surface of the [[elastomer]] [[polydimethylsiloxane]] (PDMS) was first described by the group of George Whitesides at Harvard University in 2000.<ref>{{cite journal |last1=Huck |first1=Wilhelm T. S. |last2=Bowden |first2=Ned |last3=Onck |first3=Patrick |last4=Pardoen |first4=Thomas |last5=Hutchinson |first5=John W. |last6=Whitesides |first6=George M. |title=Ordering of Spontaneously Formed Buckles on Planar Surfaces |journal=Langmuir |date=April 2000 |volume=16 |issue=7 |pages=3497–3501 |doi=10.1021/la991302l }}</ref> The gold was deposited on warmed PDMS (100 °C), and, upon cooling and the associated thermal shrinkage of the elastomer, the gold film comes under compressive stress which is relieved by creating [[buckles]]. In subsequent years, the group of John Rogers at the University of Urbana Champaign (now at Northwestern University) has developed the technology to bond very thin silicon ribbons to a pre-stretched PDMS membrane. Upon relaxation of the per-stretch, the compressive [[mechanical stress]] in the [[silicon]] ribbons is relieved by creating wavy buckles in the PDMS. As silicon is a brittle material, the ribbons need to very thin (about 100 nm) to stay intact during buckling.<ref>{{cite journal |last1=Kim |first1=Dae-Hyeong |last2=Rogers |first2=John A. |title=Stretchable Electronics: Materials Strategies and Devices |journal=Advanced Materials |date=17 December 2008 |volume=20 |issue=24 |pages=4887–4892 |doi=10.1002/adma.200801788 }}</ref>
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* '''Microcracked gold thin film''': When a thin gold film is deposited on PDMS under certain conditions,<ref>{{cite journal |last1=Graudejus |first1=Oliver |last2=Görrn |first2=Patrick |last3=Wagner |first3=Sigurd |title=Controlling the Morphology of Gold Films on Poly(dimethylsiloxane) |journal=ACS Applied Materials & Interfaces |date=28 July 2010 |volume=2 |issue=7 |pages=1927–1933 |doi=10.1021/am1002537 |pmid=20608644 }}</ref> the gold film adopts a microcracked morphology<ref>{{cite journal |last1=Lacour |first1=Stéphanie P. |last2=Chan |first2=Donald |last3=Wagner |first3=Sigurd |last4=Li |first4=Teng |last5=Suo |first5=Zhigang |title=Mechanisms of reversible stretchability of thin metal films on elastomeric substrates |journal=Applied Physics Letters |date=15 May 2006 |volume=88 |issue=20 |doi=10.1063/1.2201874 |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:41467478 }}</ref> which makes the gold stretchable. The maximum [[Strain (mechanics)|strain]] of the film decreases with the length and increases with the width of the conductor.<ref>{{cite journal |last1=Graudejus |first1=O. |last2=Jia |first2=Zheng |last3=Li |first3=Teng |last4=Wagner |first4=S. |title=Size-dependent rupture strain of elastically stretchable metal conductors |journal=Scripta Materialia |date=June 2012 |volume=66 |issue=11 |pages=919–922 |doi=10.1016/j.scriptamat.2012.02.034 |pmid=22773917 |pmc=3388513 }}</ref>
===Structural
* '''Geometric patterning, [[fractal]] patterns''': Metal traces are deposited in specific patterns, such as meandering or [[Serpentine shape|serpentine]] shapes, within a stretchable elastomeric substrate to accommodate [[Strain (mechanics)|strain]]. The resulting structure is akin to a 2-dimensional spring. The University of Ghent and IMEC in Belgium have pioneered the approach to using Meander shaped metallic structures.<ref>{{cite journal |last1=Gonzalez |first1=Mario |last2=Vandevelde |first2=Bart |last3=Christiaens |first3=Wim |last4=Hsu |first4=Yung-Yu |last5=Iker |first5=François |last6=Bossuyt |first6=Frederick |last7=Vanfleteren |first7=Jan |last8=Sluis |first8=Olaf van der |last9=Timmermans |first9=P.H.M. |title=Design and implementation of flexible and stretchable systems |journal=Microelectronics Reliability |date=June 2011 |volume=51 |issue=6 |pages=1069–1076 |doi=10.1016/j.microrel.2011.03.012 }}</ref>
** The group of John Rogers increased the maximum strain in devices created by this approach using fractal-based structures. These fractal patterns are characterized by self-similarity, i.e., a small sections of the structure yields pieces with geometries that resemble the whole structure.
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In subsequent years, the number of research papers that describes different approaches to fabricating sMEAs and their use for [[in vitro]] and [[in vivo]] research has increased immensely.
==Types and
Stretchable microelectrode arrays (sMEAs) can be categorized whether they are used with [[Cell (biology)|cells]] or [[Tissue (biology)|tissue]] slices in a dish (in vitro) or whether they are implanted in an animal or human (in vivo).
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==Applications==
===Neural
In [[neural interface]]s, sMEAs are utilized to record and stimulate [[neural activity]]. Their stretchability allows them to conform to the brain's surface or penetrate neural tissue without causing significant damage. This improves the quality of neural recordings and the effectiveness of neural stimulation, which is crucial for applications such as [[brain-machine interfaces]].
===
[[Electrocorticography]] (EcoG) with stretchable MEAs offers a less [[Invasive (medical)|invasive]] method for recording electrical activity from the brain's surface. These arrays can conform to the cortical surface, providing high-resolution, stable recordings even during brain movements. This capability is essential for applications such as [[epilepsy]] monitoring and [[brain-computer interfaces]].
===Cardiac
sMEAs are employed in [[cardiac]] monitoring and therapy. They can be wrapped around the heart to monitor electrical activity or deliver therapeutic electrical impulses. Their [[flexibility]] ensures they remain in contact with the heart's surface despite its constant motion. This application is vital for detecting and treating arrhythmias and other cardiac conditions, providing real-time monitoring and precise intervention.
===In
sMEAs are used in [[in vitro]] research to study cellular responses under various mechanical conditions. They enable the monitoring and stimulation of cells in a controlled environment, providing insights into cellular behavior and disease mechanisms. This application is particularly useful in drug testing and the development of new therapies.
===Soft
In soft [[robotics]], sMEAs create [[sensors]] and [[actuators]] that can deform in response to external forces. These applications utilize the mechanical resilience and electrical functionality of sMEAs to develop robots capable of navigating complex environments and performing delicate tasks. Soft robotic systems equipped with sMEAs can adapt to various tasks, from medical procedures to industrial [[automation]].
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